1. Field of the Invention
The present invention relates to a rotor structure of an inner rotor type motor, and more particularly of an inner rotor type motor using a rare earth magnet.
2. Description of the Prior Art
With higher performance achieved for various OA equipments in recent years, there have been strong demands for a higher output, a higher speed and lower power consumption in various motors including spindle motors, PM (permanent magnet) type stepping motors and others.
To meet such demands, use has been made of a rare earth magnet having a high energy product for a rotor magnet material. However, because of a low mechanical strength of this magnet, if a highly productive resin forming method based on a high polymer material is used as a rotor assembling method, cracks may occur in the magnet due to a forming pressure, making it impossible to maintain the function of the rotor. Under the circumstances, therefore, the only way available is a low-productive method such as adhesion or the like.
FIG. 1A shows an example of a conventional rotor structure using a magnet of the foregoing kind. A sleeve 12 is fitted in around a shaft 3 and, on its periphery, a cylindrical rare earth bond magnet 25 which is reputed to be more inexpensive than a sintered rare earth magnet is arranged in a thinned form to minimize the quantity of its use to a necessary limit whereby the magnet cannot be formed in a larger dimension, and hence is divided into two portions. If this rotor is manufactured by insert molding, two cylindrical rare earth bond magnets 25 and a sleeve 12 are set in a metallic mold and then a resin is injected. However, because of a low mechanical strength of the cylindrical rare earth bond magnet 25, cracks may occur due to an injection pressure, or even breaks may occur depending on molding conditions.
Therefore, an improved rotor structure has been proposed as shown in FIG. 1B. According to the improved structure, a rotor is manufactured in such a way that a cylindrical rare earth bond magnet 25 is beforehand fixed by adhesive K onto the periphery of a metal sleeve 26 serving as a reinforcing material, then is set in a metallic molding die together with a sleeve 12 holding a shaft 3, and the sleeves 12 and 26 are integrated by a resin 27.
The improved rotor structure has enhanced characteristics, but necessity of the adhesion step and the reinforcing sleeve inevitably leads to cost increase, which may limit application of the rotor structure manufactured as such.
The present invention was made with the foregoing problems in mind, and it is an object of the invention to provide a rotor structure of an inner rotor type motor, which uses a high-performance rare earth magnet but is low in costs.
In order to achieve the foregoing object, in accordance with the invention, there is disclosed a rotor structure of an inner rotor type motor comprising a stator including annular stator yokes each having a plurality of pole teeth along an inner circumference thereof and coils arranged inside the stator yokes, each coil being constructed by winding a magnet wire and a rotor rotatably disposed with a small gap from the pole teeth and having a permanent magnet arranged opposite to the pole teeth, wherein the permanent magnet comprises a plurality of discrete segment magnets which are integrally molded by a resin of a high polymer material and arranged apart from each other.
The permanent magnet used for the rotor comprises a plurality of discrete segment magnets, which are arranged apart from each other like a ring on the outer circumference of the rotor to keep the reduction of a surface area at a minimum limit. The segment magnets are made integral and held in place by filling a space between the segment, magnets with a resin by molding. Thus, portions holding segment magnets serve as a relief for a resin pressure caused by molding, and any cracks or breaks which may be caused in the segment magnet by a molding pressure can be prevented. In addition, reduction of the surface area for entire segment magnets is kept at the minimum limit, so there is no substantial deterioration of a torque as one of characteristics of a motor. Moreover, as the usage of magnets which are of relatively high price is minimized, reduction of cost can also be achieved.